(1) Field of the Invention
This invention relates to the partial or complete elimination of shrinkage cavities in cast ingots. More particularly, the invention relates to the partial or complete elimination of such cavities that form during direct chill (DC) casting of metal ingots, especially (although not exclusively) ingots made of aluminum and aluminum-based alloys.
(2) Description of the Related Art
Metal ingots, especially those made of aluminum and aluminum-based alloys, may be formed by direct chill (DC) casting techniques in which molten metal is fed into the upper end of a chilled annular (usually rectangular) mold as an ingot support (a so-called “bottom block”) is gradually caused to descend from an initial position closing the bottom end of the mold. The mold cools the body of molten metal in the mold around its periphery until the peripheral surface is sufficiently solid to support itself and to avoid leakage of molten metal from the hot center of the ingot. In this way, as the ingot support gradually descends, the ingot grows to a predetermined length while molten metal is continually introduced into the mold at the upper end. Cooling water is usually poured onto the surface of the ingot immediately below the bottom end of the mold to enhance the cooling process.
Once the ingot has reached its maximum length, the supply of molten metal is stopped and the ingot support remains fixed in place carrying the weight of the ingot. As the ingot cools and continues to solidify, the metal shrinks and contracts. Since the cooling commences from the peripheral surfaces of the ingot, the core of the ingot at its upper end is the last part to cool and solidify, and metal shrinkage becomes apparent from the appearance of a cavity which forms at a central position in the upper surface of the ingot. If this cavity is allowed to remain following complete ingot cooling, a portion of the upper end of the ingot is generally cut off below the cavity to provide the ingot with a flat upper surface. While the metal cut off in this way may be recycled, the procedure is nevertheless costly and inefficient. If the cavity is not removed in this way, a defect known as “alligatoring” may occur during rolling of the ingot. This involves the formation of tapered shapes (resembling the jaws of an alligator) extending from the two rolling faces of the ingot that eventually come together as rolling proceeds to form a two-layer laminate that has to be scrapped.
In the past, compensation for metal shrinkage has been provided by retaining a reservoir of molten metal above the nominal “upper surface” of the ingot so that further molten metal is available to descend into the cavity as the cavity is formed. As explained, for example, in U.S. Pat. No. 3,262,165 which issued to A. J. Ingham on Jul. 26, 1966, this can be done by providing the head of a mold with insulated walls that may be partially filled with a pool of molten metal that is kept molten by the insulation. Alternatively, shrinkage compensation may be accomplished by providing flexible hot topping liners which again provide an insulated space above the ingot for retaining a molten pool of metal. Such liners are disclosed, for example, in U.S. Pat. No. 4,081,168 which issued to R. E. Atterbury on Mar. 28, 1978. The use of such “hot tops” is not convenient for the direct chill casting process and again it may result in the need for the removal of an excess of metal from the upper part of the ingot as the molten reservoir itself cools and solidifies in contact with the ingot proper.
Ingham in the patent identified above also suggested repeated topping up of the solidifying mass, i.e. adding further molten metal to the cavity as the cavity forms. However, this solution is not generally possible in conventional direct chill casting apparatus because molten metal in the channels and spouts above the mold tends to solidify once the main casting operation has been terminated, and anyway the kind of precise control that would allow filling of the cavity while avoiding spillage has not generally been possible.
European patent application EP 0 150 670, which was published on Aug. 7, 1985 and names C. Alborghetti as the inventor, discloses a casting apparatus in which the level of metal in a mold or runner, or the like, is regulated by measuring the magnitude of eddy currents induced in the metal by means of a measuring coil, the magnitude being proportional to the distance from the coil to the metal melt. The monitoring of such distances is used in the electromagnetic casting of aluminum, but not with direct chill casting.
US patent publication no. US 2010/0032455, which was published on Feb. 11, 2010 and names Cooper et al. as inventors, discloses a control pin system for use in controlling the flow of molten metal in a distribution system for casting. The control pin controls the flow of molten metal through a spout and provides heating for the control pin or the spout to prevent solidification of metal in the spout when the flow is stopped.
Despite these disclosures, there is a need for an improved method of and apparatus for eliminating the shrinkage cavity in an ingot formed by direct chill casting.